Green’s Dictionary of Slang

flat back n.

a bedbug.

1861		G.W. Harris ‘Travels with Old Abe’ Nashville Union and American XXV Feb. in Inge (1967) 263: When he’s got his dignity on, and a passel of flat backs roun him an he feels good an safe.
1885		Dly Gaz. for Middlesborough 6 Nov. 3/4: Twenty-seven of us in one room, most of us dirty [...] Then the ‘flats’ and ‘chits’ [...] Suffice it to say that they were at (sic) work, and I was awake the whole night through.
1890–1904		Farmer & Henley Sl. and Its Analogues.